Wireless communication is an increasingly popular means of personal communication in the modern world. People are using wireless networks for the exchange of voice and data as an alternative to using a wire infrastructure. In principle, a user can seek information over the Internet or call anyone over a Public Switched Telephone Network (PSTN) from any place inside the coverage area of the wireless network.
With the increase of the popularity of wireless communications, many of the uses for a wire infrastructure are being replaced by wireless infrastructures. For example, a traditional wire infrastructure PBX may be replaced by a wireless PBX to route phone calls and Internet connections inside a building or in an office. A wireless PBX may take advantage of wireless communication techniques.
A wireless communications technique commonly used to allow multiple users on the same channel is code division multiple access (CDMA). CDMA permits multiple users to use the same wireless communication channel at the same time, which allows the network infrastructure to support more wireless users. Some of the benefits of CDMA are improved call quality, simplified system planning through the use of the same frequency in every sector of every cell, enhanced privacy, improved coverage characteristics, increased talk time for portables, and increased bandwidth.
CDMA utilizes the radio spectrum by allowing multiple users to share the same physical channel. In CDMA, multiple users occupy the same frequency at the same time. Consequently, frequency and time are not used to discriminate between users. Instead, CDMA separates multiple users using the same channel through the use of codes. The receiver in the CDMA system typically receives a waveform that consists of a mixture of signals from several users. The system then uses coding to discriminate between the signals received from the multiple users on the same frequency channel at the same time.
CDMA is a type of spread spectrum communication technique known as Direct Sequence Spread Spectrum (DSSS). In this approach, a narrowband data signal from a user is spread through the use of a broadband code that is unique to the user in order to create a broadband signal for transmission. The broadband signal is then transmitted on a frequency that may also be used by other users. When a receiver receives the broadband signal, it uses the user's unique code to recover the user's narrowband data signal from the mixture of signals encountered by the receiver.
In CDMA, each bit of the user's narrowband signal is divided into a number “m” of short intervals called chips. Each bit is typically broken down into from 64 to 128 chips. One chip corresponds to 801 feet or 244.14 m; 1 mile is 6.6 chips; and 1 km is 4.1 chips. Signals that correspond to 1 chip apart from each other have been received 640 milliseconds apart from each other due to the propagation speed of the radio waves. Each transmitting user is assigned a unique chip pattern or sequence that is, in effect, that user's code channel. Using this unique sequence, the user's transmitted signal will be distinguishable by a receiver from other signals using the same physical channel. Other user's code patterns will appear random to the receiver and will integrate in a random self-canceling fashion such that they do no disturb the bit decoding decision being made with the selected user's code pattern.
In a CDMA distributed antenna system, a number of antenna elements at different locations may simultaneously communicate on the same frequency and same channel (e.g., same Pseudonoise (PN) Offset and same Walsh code) with a given receiver. The PN offset is used primarily for signal spreading. A short PN code may identify cell sites or sectors of cells, and a long PN code may be used for spreading and scrambling a signal to provide privacy. A Walsh code contains 64 sequences, each 64 chips long. Each Walsh code is orthogonal to all other Walsh codes. A Walsh code's orthogonal nature prevents one code from interfering with other Walsh codes.
A common base transceiver station (BTS) or distributed antenna system controller (DAS) may power a group of distributed antennae used in a CDMA system. On both the forward link (communications from the base station to the receiver) and reverse link (communications from the receiver to the base stations), all of the antennas within the system may be in communication with the receiver.
Energy from a common BTS may be radiated via a DAS controller to a number of antenna elements distributed throughout an area, such as in a building for instance. A problem with this arrangement is that energy radiated via many of the antenna elements for communication with a mobile station or a receiver is largely wasted due to a redundancy of signals transmitted from the antennas within the system.